A variety of devices are used to treat fractures of long bones, many of which are disclosed in co-pending and commonly-assigned U.S. patent application Ser. No. 12/074,320, which is incorporated herein by reference. Referring to FIG. 1, one such stabilizing assembly 30 includes an intramedullary nail 31 (in this case a humeral nail), a lag screw 32 and a compression screw 33. As illustrated in FIG. 1, the compression screw 33 includes an enlarged head section 34 that, in this embodiment, bears against the humerus 35 to compress the humerus 35. A threaded section 36 of the compression screw 33 passes through the nail 31 and engages a rack 35 disposed on a side of the lag screw 32 (the rack 35 is more clearly seen in FIG. 8). In some implementations, the lag screw 32 can include a threaded portion that engages the threaded section 36 of the compression screw 33. Rotating the compression screw 33 applies an axial force to the lag screw 32, which has previously been anchored in a fragment of the humerus 35 by a threaded distal end 37. Accordingly, rotating the compression screw 33 draws the lag screw 32, and the bone fragment affixed to the end 37 of the lag screw 32, in a direction along the length of the compression screw 33 and into position for proper healing.
In another example illustrated in FIG. 2, a stabilizing assembly 40 coupled to a femur 41 includes a compression plate 42, the compression screw 33 and the lag screw 32. A head section 44 of the compression plate 42 extends into the femur 41 and supports the compression screw 33 and the lag screw 32. As described above, the compression screw 33 bears against a surface of the compression plate 42 such that rotation of the compression screw 33 applies an axial force to the lag screw 32 to draw the lag screw 32 and a bone fragment in a direction along the length of the compression screw 33.
FIG. 3 illustrates another stabilizing assembly 50 applied to a proximal tibia 51. The assembly 50 includes a periarticular plate 52, the lag screw 32 and the compression screw 33. Again, the head section 34 of the compression screw 33 bears against the plate 52 to limit further insertion of the compression screw and to provide a positive stop such that rotation of the compression screw 33 interacts with the lag screw 32 causing compression of the tibia 51.
In contrast to compression plate 42 of FIG. 2, FIGS. 4-6 illustrate a stabilizing assembly 60 which includes an antegrade femoral intramedullary nail 61, the lag screw 32 and the compression screw 33 to stabilize a fracture 63 across a femoral neck 64. The designs of the nails 31, 61, plates 42, 52, and screws 32, 33 may vary greatly and may be configured to be applicable to other parts of the anatomy not specifically illustrated here or specifically addressed in this disclosure. Regardless of the designs, the compression screw 33 interacts with the lag screw 32 to compress the bone. Alternatively, the fracture can be distracted by interaction of the compression screw 33 and the lag screw 32. Additionally, the lag screw 32 and the compression screw 32 are configured to slide as a unit within the nail or plate.
However, problems may result if bone adjacent to a fracture is weak and/or prone to damage when exposed to compression, or excessive compression, force. For example, excessive compression force could cause the femoral head 65 shown in FIG. 4 to migrate towards or into the fracture site 63 resulting in misalignment. In extreme cases, excessive compression may cause the femoral head 65 to be compressed all the way into the trochanteric region 66 of the femur 41.
Further, compressing a bone more than a recommended or intended amount may cause the lag and compression screws 32, 33 to splay apart from each other, which can inhibit or prevent the screws from sliding within the intramedullary nail 31, 61 or plate 42, 52. Thus, while applying compression force using a compression screw/lag screw system 32, 33 is an important orthopaedic technique, excessive compression can be problematic and should be avoided.
Currently, separate compression screw and lag screw drivers are utilized. To limit the compressive force transmitted to the lag screw 32, some compression screw drivers are equipped with a line indicating 0 mm and/or red line indicating to the surgeon or other use that rotation of the compression screw to apply compression force should be stopped. However, even with this type of visual aid, experienced surgeons may still apply excessive compression force across a fracture site. Hence, a more reliable system, that is less prone to operator error, and that controls bone and/or fracture compression, is needed.